Recently, non-volatile memories that are data rewritable semiconductor devices have become widely used in the storage of electronic data. In flash memories that are typical of non-volatile memories, transistors forming memory cells have floating gates or insulation films that are known as charge storage layers. In such flash memories, charges are accumulated in the charge storage layers, so as to store data. Studies are still being made to produce non-volatile memories with higher storage capacity and density.
U.S. Pat. No. 6,011,725 discloses a SONOS (Silicon Oxide Nitride Oxide Silicon) flash memory that has virtual-ground memory cells, with each cell interchanging the source and drain, and operating the source and drain in a symmetrical fashion. In this flash memory, bit lines that also serve as the source and drain are formed in the semiconductor substrate, and charges can be accumulated in the trapping layer in an ONO (Oxide Nitride Oxide) film formed on the semiconductor substrate. By interchanging the source and drain, two charge storing regions can be formed in one memory cell. Accordingly, higher storage capacity and density can be achieved.
In the conventional structure disclosed in U.S. Pat. No. 6,011,725, however, the two charge storing regions overlap with each other if the distance between any of the two bit lines is made as short as 100 nm or less. Also, a substrate current flows between the bit lines on the substrate side, which is called “punchthrough.” Therefore, it is difficult to shorten the distance between the bit lines, and there is a limit to the amount the storage capacity and density of each memory cell can be increased